1) Field of the Invention
The present invention relates to an image display apparatus having an observation optical system.
2) Description of Related Art
In recent years, development has been energetically made for image display apparatuses, specifically for those to be held on the head or face of individuals for entertaining them with a wide-screen image. Conventionally, as head-mount image display apparatuses, there are known a type in which an image on an image display element such as a CRT is transmitted via an image transmitting element to an object surface, where the image is projected in the air by a toric reflecting surface (U.S. Pat. No. 4,026,641), and a type in which an image displayed on a liquid crystal display device (LCD) is once imaged in the air via a refraction-type relay optical system and then is introduced into an eye of an observer via an eyepiece optical system composed of a concave mirror arranged in a decentered manner (Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 6-294943).
However, these types of the head-mount image display apparatus are not suitable for long-time use as being held on the head or face, because their optical systems are large and heavy. Therefore, it is desired to reduce the weight of the apparatus while maintaining good image quality.
Also, in accordance with recent popularization of cellular phone and portable intelligent terminal, requirements for wide-screen view, via an image display apparatus, of graphics or text data on a cellular phone or portable intelligent terminal have grown. For application to a cellular phone or the like also, it is desired to reduce weight of the apparatus while maintaining good image quality because users would personally carry the cellular phones or the like with them in their pockets or bags.
As conventional resolution examples for achieving weight reduction, observation optical systems that are constituted with a combination of thin platelike optical members (Japanese Patent Application Preliminary Publication (KOKAI) No. Hei 8-234137, Japanese patent Application Preliminary Publication (KOKAI) No. 8-240773) are known.
Although such an observation optical system is allowed to have a lightweight structure, it involves a problem in having difficulty in providing good image quality, because dust is easily caught on reflecting surfaces thereof to cause flare or the like.
In reference to FIGS. 29–30, this problem is explained. Each of observation optical systems shown in FIGS. 29, 30 is configured to reflect bundles of rays emergent from an image display element 1 such as LCD at a reflecting surface 21 of an optical member 2. Since the observation optical system as shown in FIG. 29 uses a prism, there is no chance that dust or flaws would damage the reflecting surface 21 during assembling or transportation. In contrast, since the observation optical system as shown in FIG. 30 is constituted with a combination of thin platelike optical members for the purpose of weight reduction, the reflecting surface 21 is apt to be damaged with dust or flaws and thus assembling and transportation is difficult.